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A sense of building tension. Apparent is the chilled vapour coming from my fellow runners as they do the "about to run" wiggle on the starting line.

A quick twitch of the hips.

I'm participating in my local weekly parkrun event and I'm focused. The countdown begins...

Oh, hang on. I have to sort my device. So does the guy next to me.

The person behind us sighs. I won't start until… done! GPS ready, and I'm running.

During the scramble, it's challenging to see a wrist without a fitness tracker on it. And this isn't only us runners who rely on them.

This is a booming, massive industry. Brands like Garmin are prevalent, featuring an broad range of different devices to suit a range of needs. Based on how advanced the model you go for, prices can start from £100 to thousands of pounds.

"I find it infuriating, I'm unable to relax."

"It provides solace… It's like a supporter supporting me."

"Stop telling me I've had a poor sleep."

Just some of the responses people have shared from fellow tracker enthusiasts when I ask what they their opinion is on theirs.

The Data Dilemma

My thoughts on mine? Currently, annoying. It's telling me I'm off pace, leading me to doubt whether the foliage I'm running below are interfering with my location tracking.

Countless individuals of us attach these little devices onto our wrists and use them constantly, ostensibly unconcerned with the fact they're constantly tracking us.

Long past are the days when we relied on them only to give us an rough count of our step count. Now, they can measure our sleep patterns, stress levels, heart rate variability (HRV), our VO2 max.

It's a challenge to find a physical metric they fail to monitor.

But are they able to do what they claim to do? And are the numbers we scrutinise a faithful picture of reality? Do they boost our daily wellness, or do the plethora of readings and constant notifications just increase our everyday anxiety?

Addiction or Aid?

"I do get a a little obsessed with mine," a wearer reveals.

While she appreciates how her device tracks her workouts, she finds some of its other features "overwhelming" at times - a feeling first discovered during a pregnancy.

Before she realised she could adjust the settings, her smartwatch kept telling her she was not being active enough. In a new phase, the watch insists her she's had a poor sleep score.

"I am caring for a six-month old, I don't need to be informed I'm tired," she says. "I'm acutely aware perfectly well."

Is it not possible to just remove it?

"It's possible, theoretically, but I have this complicated dynamic with it," she explains. "I love it for its workout tracking, but I'm unsure whether the myriad other functions it can now do is possibly overkill for me."

Inside the Device

Different brands of smartwatch has its own novel way of measuring your bodily functions and processing your data, but the most use sensors on the reverse of the watch.

They commonly project small, green LED lights onto your skin which can detect pulsation, recording your pulse rate. Higher-end devices detect changes in the electrical current that runs through the skin to assess your nervous system activity.

Expert Perspectives

A professor says that in general for smartwatches, it's a "delicate equilibrium" - while the continuously improving tech could realistically aid early diagnosis, "identifying issues before we show symptoms", it could also turn us watch wearers into the "concerned but fine".

He points out that the most advanced wearables can carry out tests like heart rhythm analysis, which regularly monitor how healthy the heart is. They can alert if someone is experiencing atrial fibrillation (AF), where there is abnormal signals in the heart causing an arrhythmia.

This does not necessarily indicate someone is imminently going to suffer a heart attack, but it can give an preliminary indication they might be more at risk of a blood clot in the future.

But how to interpret these numbers is not straightforward. The expert concerns that as further capabilities are added, people may find it hard to fully understand their data.

"I have some doubts that having the capacity to monitor so many metrics is such a positive development," he says.

A health researcher focusing on patient behaviour concurs. A study into the effect of wearable tech on a set of individuals with health issues suggested a minority of those who were given devices to monitor their condition experienced increased anxiety and were "far more likely to use healthcare resources".

She observed a cycle with her patients - they noticed a alarming metric on their watch. They felt anxious. Their BPM went up. They got increasingly anxious. They checked again, and their heart rate rose further.

"Upon viewing stats about ourselves that we don't really understand then, of course, we are going to seek clarification," the psychologist says. "We look, we re-check - it becomes a vicious cycle."